Since the beginning of the electronic era, the primary building blocks in electronics and microelectronics have been field-effect transistors (FETs) based on inorganic electrodes, insulators, and semiconductors. These materials have proven to be reliable and highly efficient, providing performance that improves continually according to Moore's law. More recently, organic materials have been developed as both active and passive materials in electronic circuitry. Instead of competing with conventional silicon technologies, organic FETs (OFETs) based on molecular and polymeric materials are desired in niche applications, for example, in low-end radio-frequency technologies, sensors, and light emission, as well as in integrated optoelectronic devices such as pixel drives and switching elements in displays. These systems have been widely pursued for the advantages they offer, which include processability via vapor/solution-phase fabrication, good compatibility with different substrates (e.g., flexible plastics), and opportunities for structural tailoring. This trend is further driven by the continued demand for low-cost, large-area, flexible and lightweight devices, and the possibility to process these materials at much lower substrate temperatures compared to inorganic semiconductors.
The simplest and most common OFET device configuration is that of a thin-film transistor (TFT), in which a thin film of the organic semiconductor is deposited on top of a dielectric with an underlying gate (G) electrode. Charge-injecting drain-source (D-S) electrodes providing the contacts are defined either on top of the organic film (top-configuration) or on the surface of the FET dielectric prior to the deposition of the semiconductor (bottom-configuration). The current between the S and D electrodes is low when no voltage (Vg) is applied between the G and D electrodes, and the device is in the so called “off” state. When Vg is applied, charges can be induced in the semiconductor at the interface with the dielectric layer. As a result, current (Id) flows in the channel between the S and D electrodes when a source-drain bias (Vd) is applied, thus providing the “on” state of a transistor. Key parameters in characterizing FET performance are the field-effect mobility (μ), which quantifies the average charge carrier drift velocity per unit electric field, and the current on/off ratio (Ion:Ioff), which is the D-S current ratio between the “on” and “off” states. For a high-performance OFET, the field-effect mobility and on/off ratio should both be as high as possible, for example, having at least μ˜0.1-1 cm2V−1s−1 and Ion/Ioff˜106.
Most OFETs operate in p-type accumulation mode, meaning that the semiconductor acts as a hole-transporting material. For most practical applications, the mobility of the field-induced charges should be greater than about 0.01 cm2/Vs. To achieve high performance, the organic semiconductors should satisfy stringent criteria relating to both the injection and current-carrying capacity; in particular: (i) the HOMO/LUMO energies of the material should be appropriate for hole/electron injection at practical voltages; (ii) the crystal structure of the material should provide sufficient overlap of the frontier orbitals (e.g., π-stacking and edge-to-face contacts) to allow charges to migrate among neighboring molecules; (iii) the compound should be very pure as impurities can hinder the mobility of charge carriers; (iv) the conjugated core of the material should be preferentially oriented to allow charge transport in the plane of the TFT substrate (the most efficient charge transport occurs along the direction of intermolecular π-π stacking); and (v) the domains of the crystalline semiconductor should uniformly cover the area between the source and drain contacts, hence the film should have a single crystal-like morphology.
Among the organic p-type semiconductors used in OFETs, the classes of (oligo, poly)thiophenes and acenes are the most investigated. For instance, the first report on a polyheterocycle-based FET was on polythiophene, and poly(3-hexyl)thiophene and α,ω-dialkyloligothiophenes were the first high-mobility polymer and small molecules, respectively. Over the years, chemical modifications of the π-conjugated core, variations in ring-to-ring connectivity and substitution pattern have resulted in the synthesis and testing of a considerable number of semiconductiong materials with improved mobilities.
In order to take full advantage of the cost effciencies of solution processing methods such as spin coating, stamping, ink-jet printing or mass printing such as gravure and offset printing, polymeric organic semiconductors would seem to be the material of choice. Among polythiophenes, soluble regioregular polythiophenes such as poly(3-hexylthiophenes) (P3HT), or poly(3,3′″-didodecylquaterthiophene), poly(2,5-bis-(3-dodecylthiophen-2-yl)-thieno-(3,2-b)thiophene, poly(4,8-didodecyl-2,6-bis-(3-methylthiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene) and their variants are most promising for OTFT applications due to their high charge carrier mobilities. See for eg. Ong, B. S. et al. J. Am. Chem. Soc. 2004, 126, 3378-3379; McCulloch, I. et. al. Nat. Mater. 2006, 5, 328-333 and Pan, H. et. al. Adv. Funct. Mater. 2007, 17, 3574-3579.
Despite recent advances one of the major drawbacks of these polymers is the need for post deposition annealing to achieve their high mobilities. The annealing temperature can range from 120° C. to 200° C. for between 15 minutes to a few hours. For flexible electronics, if the annealing temperature of organic semiconductor is higher than the glass transition temperature or melting point of the plastic substrate, then the substrate softens before the semiconductor mobility is optimized. Further, adapting even relatively low annealing temperatures for a prolonged period of time in a reel to reel process involves significant costs and lower throughput.
Another drawback with the state-of-the-art high performing semiconductors is the poor solubility in common organic solvents at room temperature. These polymers are sufficiently soluble only in high boiling point chlorinated solvents such as dichlorobenzene and sometimes only at elevated temperature.
Hence, for reel to reel, low cost production of organic electronics, polymeric semiconductors that can be formulated in reasonably high concentrations in common organic solvents and that do not require high and extensive annealing are necessary.
Vinylene moieties in polymers are advantageous as they reduce the aromaticity of the backbone and hence improve charge delocalization, leading to lower band gaps, better conjugation and improved charge transport. Also, the incorporation of vinyl moiety in the semiconductor backbone is expected to afford a certain degree of rotational freedom between its neighbouring aromatic units, which should a priori help to improve the solubility and hence processability of the polymer and, further, to reduce the energy requirement for molecular packing in the solid state (annealing temperature/time). This, in turn offers advantages in fabricating solution processed electronic components such as OTFTs, OLEDs and OPVs.
However, the use of vinyl moieties in polymers has been limited to poly(phenylene vinylenes) and poly(thiophene vinylenes) (PTVs) and variants which have been synthesized and developed. Among the earliest reports of semiconducting polymers are poly(para-phenylenevinylene)s (PPVs) and their derivatives used as active materials in organic light emitting diodes (OLEDs). See eg. Burroughes, J. H. et al. Nature 1990, 347, 539-541 and Kraft, A. et al. Angew. Chem. Int. Ed. 1998, 37, 402-428. PPVs have a relatively large band gap, and poor hole mobilities. For this reason, PTVs and its derivatives were adopted for use in OTFTs. See eg. Fuchigami, H. T. et al. Appl. Phys. Lett. 1993, 63, 1372; Prins, P. et. al Adv. Mater. 2005, 17, 718; Gillissen, S. et al. Synth. Met. 2003, 135-136, 255 and Yamada, S. J. Chem. Soc., Chem. Commun. 1987, 1448. It is expected that the high proportion of the vinyl bonds along the polymer backbone makes these polymers disordered in the solid state and this results in the observed hole mobilities of only 10−4-10−2 cm2/Vs.